||[Jun. 11th, 2010|11:15 pm]
Why when I turn my projector off, does the fan spin up for a few minutes? My first thought was that it doesn't make sense, because it's not getting any hotter what with the power being off. Furthermore, cooling it off fast will only heat-cycle it harder. On the other hand, maybe the filliment is soft; maybe this is protecting it from the once in a blue moon that I'll move the thing right after watching? My only other thought is that there is some component that would get hotter before it got cooler if the fan didn't stay on, but that (a) sounds bogus and (b) seems like a design way too close to the edge and (c) doesn't explain why the fan would have to spin faster for cool-down thanfor steady-state projecting.
2010-06-12 04:11 am (UTC)
Might it just be so that the cooldown happens fast enough that people don't freak out and decide it is buggy and unplug it? It does seem to be a ubiquitous design, there really ought to be a reason.
Well, as near as I can determine, it is done for two reasons - first, a hot filament is, in fact, softer and more prone to breaking when subjected to vibration, such as moving. Rapidly cooling the lamp helps to mitigate that. Secondly, spinning up the fan serves as an audible "do not disturb" cue so that people don't jostle it when the lamp is hot.
Also, even though the lamp is no longer actively emitting, there is passive heat soak that happens as the heat sinks attached to the lamp housing use the lamp base and filament as a heat sink, so the heat flow is reversed, and needs active cooling to have its effects mitigated.
My (1983-vintage) car does this, too. It has a cooling fan that only comes on when the car is turned off, and is thermostatically (we assume) controlled.
Don't know why. Unplugging it made no noticeable difference to the car's behavior.
I think I found the answer (after watching How It's Made
on assembling HID lamps). My theory is that it's a form of arc lamp
(e.g., a metal halide lamp
). Apparently some are
while some are
. I don't understand the chemistry of this, but it looks like cold restrike bulbs are probably cheaper and that restriking them while hot either reduces their life or requires a higher voltage. It would be interesting to know the details, but if this is the case it would completely explain why the projector spins up the fan and why it won't let me turn it back on while it is in its cooldown phase. (It does restart if I unplug it and plug it back in, which makes it loose state, so I clearly it can